Document, improve ability to provide own plungs

[aendra-rininsland]

Currently if you provide the FERRET_PLUGINS_DIRECTORY it will override the default behaviour, meaning you have to have all the default plugins in a local directory if you want to set that.

1. FERRET_PLUGINS_DIRECTORY isn't documented anywhere AFAICT
2. If provided, it should merge with the default plugin directory, not override it

[ejb]

+1 for plungs